When using a soldering tool or other high temperature device it is sometimes necessary to measure resistance in the heated element (e.g., soldering tip). If the tip-to-ground resistance of a soldering tip increases, the likelihood of an electrostatic discharge between the soldering tip and the article being soldered also increases. This electrostatic discharge could damage the article being soldered, especially for active devices composed of CMOS, GaAs, or other semiconductor materials. Department of Defense specifications require that a maximum of 2 ohms resistance be allowed between the soldering tip and the ground connection of the energized soldering tool.
A known method for measuring resistance in a soldering tip or similar heated element requires the use of a standard d.c. current source and the measurement of the voltage from tip to ground. The magnitude of the resistance is then computed by dividing the measured voltage value by the magnitude of the current source. This method leads to several errors. Electrical noise, for example, generated in the hot soldering tip, will cause an increase in the measured voltage. This increase in voltage may have a magnitude of several millivolts a.c. Another error found in this environment is a thermocouple-type voltage that appears at the measurement junction, a phenomenon commonly known as the Seebeck effect. Depending upon the polarity of the Seebeck effect, voltage will be either added to or subtracted from the measured voltage.
In order to cancel the Seebeck effect in the prior art methods, the polarity of the current going through the soldering tip must be manually reversed and a second measurement of the voltage from tip to ground be made. The two measurements are then averaged, and the average value is divided by the magnitude of the current to compute the tip-to-ground resistance. This method is cumbersome and time-consuming. Also, the noise in the activated tip will still appear in the final resistance measurement. This method suffers from inefficiencies in time and accuracy in measuring resistance in an active, high temperature environment.